Multicast communication is an efficient way to send data over a computer network from a single source to a plurality of hosts or destinations. Multicast data, in the form of multicast packets, is sent from a source (S) address to a group (G) address. Multicast routing comprises two functional components. One component is a Reverse Path Forwarding (RPF) check to verify the interface on which the packet is received using information in a topology table. The other component is replication which is to forward the packet to interface(s) described by entries in a routing table.
In unicast routing, unicast packets are sent from a single source to a single host or destination. The packets are routed by network devices, such as routers, towards a destination address (typically and IP address). The source address of a unicast packet therefore plays little or no part in routing of unicast data, with the unicast packets being routed based on their destination address.
In Multi-Topology Routing (MTR), the forwarding table chosen to route an incoming packet is determined by DSCP (Differentiated Services Code Point) bits in the packet header. Unicast routing protocols (e.g. OSPF and ISIS) are enhanced to build different topologies by incorporating “color-marking” in a route advertisement.
There are certain limitations when this model is extended to multicast routing. For example, multicast routing states can be created by end hosts joining and/or leaving a group. It is not practical to modify the operating system on these systems to include “color” in the IGMP (Internet Gourp Management Protocol) packets. Therefore, last-hop routers are restricted to supporting only one multicast routing table for a given group or a given source/group pair. This in turn makes it very complex to define multiple topologies in the transit routers for the same group or source/group pair.
A somewhat different difficulty applies in a MVPN (Multicast Virtual Private Network). In the current MVPN environment, for a given source, the topology table can be obtained from one and only one VRF (Virtual Routing and Forwarding) device. With the restriction, if a source sends multiple multicast streams to different MVPNs, and if there are hosts or receivers interested in different combination of the streams, the source has to use a unique IP address per stream to deliver the traffic. This introduces additional management overhead.